1. Field of the Invention
The present invention relates to novel blowing agents for platinum-catalyzed organopolysiloxane compositions of the SiH/vinyl-Si type, which may be converted into silicone foams at ambient or higher temperatures, and which foams are useful, e.g., for insulation purposes.
2. Description of the Prior Art
Presently, the silicone foams are typically prepared according to one of the following three methods:
(1) a blowing agent is added, that is to say, an additive which produces a gas, frequently nitrogen, when heated: a method of this type is described particularly in U.S. Pat. Nos. 3,425,967, 3,429,838, 3,677,981 and 4,189,545. Blowing agents of this kind are, for example:
azobis(isobutyronitrile), PA0 dinitropentamethylenetetramine, PA0 terephthalazide, PA0 azodicarbonamide, and PA0 water,
(2) a gas is incorporated into the organopolysiloxane composition at atmospheric pressure when the components of the composition are being mixed, the mixture gelled under vacuum, and then the silicone is cross-linked (U.S. Pat. No. 4,368,279),
(3) a gas, usually hydrogen, is generated, in situ, when the composition is being mixed. The release of hydrogen is produced by the reaction of an organohydrosiloxane with a hydroxyl group. The compositions then generally contain silanol and/or alcohol groups. Compositions of this type are described particularly in U.S. Pat. Nos. 3,070,555, 3,338,847 and 3,923,705. However, the presence of hydroxyl groups (silanol, alcohol) is not necessary, it being possible for such groups to be formed, in situ, by the reaction of water with organohydrosilanes (U.S. Pat. No. 3,516,951).
The first method has the disadvantage of requiring heating, generally to a fairly high temperature (typically above 180.degree. C.) in order to produce satisfactory results.
The second method requires apparatus for the application of vacuum and, consequently, its use on an industrial scale is restricted to the manufacture of articles of limited size and using a multistage process.
The third method, which is the most widely used, produces hydrogen, which is somewhat hazardous when placed in ovens to effect crosslinking.